The present invention relates to the sampling and analysis of gases dissolved in liquid, and more specifically to the vivo analysis of gases dissolved in the blood.
Blood gases have been detected and measured in vivo by a variety of electronic means. The most successful have been variations of the polarographic electrode for oxygen and modified pH electrodes for carbon dioxide. Oxygen, CO.sub.2 and other dissolved blood gases have also been detected and measured in the prior art on the basis of their flow rates into an evacuated gas permeable membrane tipped catheter in contact with the blood. The prior art systems of the latter type are specifically designed to operate with mass spectrometers. Blood gases pass through a small membrane area and are drawn to the mass spectrometer at a rate proportional to their partial pressures in the blood. The mass spectrometer determines the relative number of each type of gas molecule passing into the system and thus, with proper calibration, the partial pressures in the blood may be indirectly determined.
All of the prior art methods described here rely on the rate of gas diffusion through a membrane to indicate the partial pressures of the gases in the blood. In such measurements, a steady state diffusion rate is reached which is a function of the membrane thickness, the membrane surface conditions, blood velocity, temperature, etc., which parameters are either unknown or difficult to control. The combined effects of these variables on the overall measuring system can only be overcome by calibrating each system after it is in place in the artery. It is also known that these variables may change during the time that a continuous blood gas measurement is being made. The membrane probes are known to change position within the blood stream resulting in varied blood flow conditions which can alter the gas diffusion rates. The membrane characteristics will also change under the effect of protein buildup on their surfaces, thus changing the diffusion rates. It is thus necessary to calibrate such systems frequently during their use to account for the changes in gas diffusion rates which naturally occur. Each calibration necessitates the extraction of a blood sample for gas pressure determination with an in vitro instrument.